Method and apparatus for improved heat extraction from aluminum castings for directional solidification

ABSTRACT

Method and apparatus for improving the quality and mechanical properties of an aluminum alloy engine cylinder block or other large or complex castings by providing sand molds bound with a soluble binder only at locations on said casting from which rapid cooling for directional solidification and/or improved localized mechanical properties are desired with said molds being otherwise bound at the remaining locations only with a more typical insoluble binder.

RELATED APPLICATION

Benefit is claimed of the prior filing date of provisional applicationNo. 60/662,192, filed Mar. 16, 2005 in accordance with 37 CFR §1.78(4)and 35 USC §119(e).

FIELD OF THE INVENTION

The invention relates to production of aluminum alloy castings,particularly the production of relatively large and/or complex castings,such as high-quality aluminum cylinder blocks for automotive engines,using sand molds.

BACKGROUND OF THE INVENTION

It is known from U.S. Pat. No. 5,297,611 and its division, No.5,477,906, to produce aluminum-alloy cylinder blocks wherein a thermalgradient is formed in the cast piece during cooling to promotecontrolled solidification of the liquid aluminum-alloy within a sandmold by the utilization of heat-conductive inserts (which function asheat sinks and in the industry are generally called “chills”). Tofunction as a heat-sink, the chill plate has mass (usually of iron) thatat least begins at a temperature lower than the solidificationtemperature of the aluminum alloy. It thus promotes early solidificationof the block starting at the surface of the chill in contact with thenewly cast block. The chill is typically placed so that thesolidification is directed to proceed in a direction towards the sourceof molten metal, usually at the opposite end of the block. This avoidspremature solidification in areas that would block access to the sourceof molten metal (which blockage prevents filling in detrimental voidsthat can otherwise be caused by the shrinkage resulting from coolingduring solidification of the casting). The utilization of such chillsaids in producing high-quality engine blocks, because the liquidaluminum solidifies in a more orderly manner thereby helping toeliminate such voids and associated shrinkage porosity which often occurwhen the block is allowed uncontrolled solidification in all directions.

Although there are suggestions in the prior art that the chill orthermal core can be brought into contact with an external heat sink orother heat extraction means so as to maintain a continuing heatextraction from the casting throughout the solidification step, saidpatents are vague with respect to a practical way of achieving such heatextraction. They refer to some means for continuously removing heat fromthe solidifying melt to thereby develop and maintain the strong thermalgradients necessary to achieve directional solidification, and teach twogeneral ways of achieving this cooling: (a) by increasing the heatextraction area of the chill by providing it with cooling fins (whichthen may be contacted with forced cooling air or mist, as needed); and(b) by providing a channel through said chill for allowing water to becirculated to cool the chill.

These and other currently-used chills do provide means for promotingdirectional solidification. However, the applicants have discovered thatmore rapid and better-controlled directional solidification, withconsequent improved quality of the castings, can be achieved bycarefully controlled selective direct water impingement on specificareas of the solidifying casting. This is accomplished in part byutilizing sand molds and/or cores that are formed with a water-solublebinder which are at least partially removed by jets of water to cool andquickly begin to play directly on the selected newly-solidified metalskin of the casting, thus resulting in a greater thermal gradient in anda more rapid cooling of the casting, whereby the solidification of theblock is thereby improved and strongly driven in the desired direction.

Part of this improvement is well described in the recently publishedU.S. patent application No. 2004/0050524 A1 (filed Mar. 18, 2004 andentitled “Mold-Removal Casting Method and Apparatus”). This and allpatents or other documents cited in this text, and all documents citedor referenced in the documents cited in this application as filed, areincorporated herein by reference. Documents incorporated by referenceinto this application or any teachings therein may be used in thepractice of this invention.

However, there remain significant drawbacks to the teaching anddisclosure in the 2004/0050524 publication. The water soluble binderstypically are of higher cost and may have less desirable moldingattributes. In addition, it can be difficult to control the specificapplication of the cooling water (or solvent) to precisely defined anddelimited areas of the complex casting (needed to achieve the mosteffective control over the site of the cooling to give the best anddirectional accuracy of cooling and thereby of the controlledsolidification progression therefrom to obtain the desired high qualityresults).

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to provide new processes andapparatus for casting aluminum alloys engine cylinder blocks in sandmolds with even better quality and for overcoming the drawbacksencountered in currently known processes.

It is a further object of the present invention to provide new processesand apparatus for casting aluminum alloys engine cylinder blocks in sandmolds wherein the solidification of the liquid metal melt is directed toproceed in a predetermined direction by enhancing the heat extraction ofthe casting undergoing solidification by contacting precise portion(s)of said casting with a fluid cooling agent.

Other objects of the invention will be pointed out or will be evidentfrom the following description of the preferred embodiments and theaccompanying drawings.

Method and apparatus for improving the quality and mechanical propertiesof an aluminum alloy engine cylinder block or other large or complexcastings by providing sand molds bound with a soluble binder only atlocations on said casting from which rapid cooling for directionalsolidification and/or improved localized mechanical properties aredesired with said molds being otherwise bound at the remaining locationsonly with a more typical insoluble binder.

The present invention comprises new processes and apparatus forimproving the quality and mechanical properties of an aluminum alloyengine cylinder block or other large or complex castings by providingsand molds bound with a soluble binder only at locations on said castingfrom which rapid cooling for directional solidification and/or improvedlocalized mechanical properties are desired with said molds beingotherwise bound at the remaining locations only with an insolublebinder, and in the process proceeding to remove initially only thoseportions of the sand mold bound with the soluble binder by contactingsuch portions with a stream of solvent, normally water, whereby saidcooling solvent removes the soluble portions only and contacts saidcasting accurately and selectively thus providing a focused cooling withresulting improved localized control while retaining the benefits ofsuperior quality and less expense typically derived from use ofwell-established insoluble binders.

The relatively low heat conductivity of the insoluble sand mold portionsremaining in place can advantageously be used to protect those parts ofthe cooling casting that should have their solidification retardedrelative to the remainder of the casting.

Additionally, the insoluble portions can be positioned at strategicpoints to support the casting while hardening, while permitting removalof sufficient other portions of the mold to accelerate the cooling (allwithout danger of slumping or distortion of an insufficiently supportedcooling casting).

Careful shaping of the relative soluble and insoluble portions relativeto one another and/or relative to the solvent stream and the castinghelp to further enhance the control over the cooling, such as byfunneling the solvent stream to and timing of the erosion of the solubleportions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a front view of an aluminum alloycylinder engine block with sand mold and cores illustrating a preferredembodiment of the present invention.

FIG. 2 is a schematic diagram of a side view of one of the water solublecore portions of FIG. 1, showing a plurality of cooling water jets eachdirected into a respective preformed hollow in said core portion readyto wash away said core portion (so as to quickly begin impingingdirectly on the block in the initial stage of solidification, but with adelay sufficient to assure needed support to the molten metal until thecooling has sufficiently solidified the surface of the casting to beself-supporting).

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to FIGS. 1 and 2, the numeral 10 designates generally a sandmold where the cylinder block is cast by filling the mold cavity withliquid aluminum alloy through a low pressure casting process.

Although the preferred embodiments of the invention are described belowas applied to an aluminum alloy cylinder engine block casting and itsmold used with a low pressure casting process; it will be understoodthat it may be also applicable to other types of castings and castingprocesses.

Similarly, in the preferred embodiments discussed below, the solvent isdescribed as being water and the soluble binder is understood to bewater-soluble; but other solvents and binders soluble therein may beused within the broader aspects of this invention.

The sand mold 10 has a cope portion 12 and a drag portion 14. In thisembodiment, the cope and drag are made with an insoluble binder. Aplurality of sand cores made with water soluble binder are set in apredetermined arrangement within said sand mold 12 and altogether definea casting cavity which is to be filled with liquid aluminum alloy toform the cylinder engine block 16.

A silica sand core 18 made with a water-soluble binder is placed inthose areas of the aluminum block where a rapid cooling is desired. Inthe embodiment of the invention herein illustrated the portions of theblock where a rapid cooling is desired at least include those portionsclose to the crankcase designated with numeral 24. This is not only toachieve directional cooling control from the crankcase area 24, but mayalso be to give enhanced hardening at that area 24 by a more rapidcooling and thereby to minimize precipitation of the hardeningingredient in the aluminum alloy.

A plurality of nozzles 20 direct water jets against the core 18, whichis bound with a water-soluble binder, and cause the destruction toremove said corel 8. Suitable cavities 26 are optionally made in thecore 18 to shorten the time needed for the water to dissolve the water-soluble binder of core 18. Thickness of the core 18 at the cavities 26,together with the at least temporary support structure 27 remainingbetween adjacent cavities, is sufficient to withstand the weight of theliquid aluminum filling the mold 10 and at the same time is as small aspossible in order to facilitate the fast destruction of core 18 toenable the water to contact the aluminum alloy as soon as sufficientinitial solidification has been achieved at the exposed area ofimpingement.

In another embodiment, the invention can be applied specifically toobtain better mechanical properties in some desired areas of the castblock, for example, in those areas where bolts will be placed forfastening other motor components to the block. One such area of theblock is illustrated with numeral 28. A water-soluble core 30 is placedcontacting such area 28 so that when a water jet is directed againstsaid core 30, the binder is dissolved and the sand washed away and,therefore, the water rapidly contacts the block metal undergoingsolidification. This quenching of the block at that point produces aparticularly rapid cooling that improves the mechanical properties inthe general area of impact relative to more remote areas in the mass ofthe casting, where cooling is less rapid but the mechanical propertiesare not as critical.

Binders with varying degrees of solubility can be used for even greatercontrol. For example, core 18 can be formed with a binder that takeslonger to dissolve than the binder used for core 30 (when subjected tothe same conditions).

Though described as a core, the insert 30 can also be considered asbeing part of the cope portion 12 (likely differing only in its shapeand its binder).

Having part of the mold be made with an insoluble binder allows otheradvantages; such as alternatively forming the core 30 in the shape of a“V” (with the apex engaging the casting). This permits the jet ofcooling water to flow in one leg of the V and out the other leg. This ishelpful in a narrow passage (and also aids in sweeping away any blanketof blocking steam that might tend to form, slowing the cooling processat the desired area 28). The generally V shape should be understood tobe broadly inclusive of a U shape as well.

Jets of water can be directed at both legs of the V initially, and thenone such jet can be shut down, once the two legs have been cleared ofthe sand in the V, to allow an unobstructed sweep of water through the Vby the continued flow from the other jet.

The concept of the V can be more broadly applied; such as by having afirst mold portion (formed with a soluble binder) in the shape of aninternal core (such as similar to a water jacket cavity) with inlet andoutlet legs accessing such core and set in a surrounding second moldportion (formed with an insoluble binder). The legs and core can haveany shape, so long as it is feasible for the timely and effectiveremoval of the sand and soluble binder by jets of solvent. The removalof the sand in said legs by jets of water result in flow passagesthrough the second portions that help to focus and aid the flow ofcooling water to the defined core area for accelerated cooling.

In a further embodiment of the invention in one of its broader aspects,the mold can be shaped to have only the portion(s) with insoluble binderforming the mold cavity, but having the portions thereof which areadjacent to the areas where rapid cooling is desired to be very thin andto be backed up by a supportive layer of mold portions formed withsoluble binder. This would give a greater uniformity of surface in theresulting casting, while allowing cooling water to displace the sandwith soluble binder early on and thus to rapidly play on the thinportions to initiate early directional cooling at the desired areas.

As used in this application, a “large complex casting” is used to mean acasting which is of sufficient size and/or complexity to makedirectional cooling a necessity to avoid voids or shrinkage porosity inthe produced casting (so as to prevent a resultant,commercially-unacceptable, large number of defective castings).

1. A sand mold for defining a cavity for forming an aluminum alloycasting, comprising at least one first portion formed with a bindersoluble in a given solvent and at least one second portion formed withbinder insoluble in said solvent.
 2. A sand mold according to claim 1,wherein said soluble first portions are each externally accessible to aflow of solvent and also are positioned at a respective location todefine an area of the casting cavity where relatively rapid cooling ofthe molten casting is desired; with one or more second portionspositioned to define the remainder of the casting cavity.
 3. A sand moldfor a large complex casting according to claim 2, wherein the solvent iswater.
 4. A sand mold for a large complex casting according to claim 2,wherein at least one first portion formed with a soluble binder has apreformed hollow shaped to receive a dissolving and cooling flow ofsolvent with a hollow depth accelerating removal of the respective firstportion from the casting surface while initially supporting theinitially molten poured casting at said area until the casting at sucharea is self-supporting.
 5. A sand mold for a large complex castingaccording to claim 2, wherein at least one of said insoluble secondportions is positioned and shaped to help focus and aid the flow ofcooling water to at least a respective one of the defined areas forrapid cooling.
 6. A sand mold for a large complex casting according toclaim 5, wherein at least one first portion formed with a soluble binderhas a generally V-shape
 7. A sand mold according to claim 1, whereinsaid soluble first portions are each externally accessible to a flow ofsolvent and also are positioned at a respective location whererelatively rapid cooling of the molten casting is desired, wherein saidsecond portions define the entire area of the casting cavityattributable to the sand mold, wherein said second portions at theaforesaid locations are thinner so as to allow more rapid coolingthereacross while still presenting a uniform surface to the mold cavity,and wherein said second portions at the aforesaid locations are backedup structurally by respective first portions.
 8. A casting apparatus,comprising: a source of molten metal; a mold according to claim 2 forreceiving molten metal from said source; a source of said solvent; andat least one nozzle positioned to spray solvent from its source toimpinge on a respective one of said first portions, formed with asoluble binder, for washing away said first portion to impinge upon andrapidly cool said casting at the resulting area of impingement.
 9. Anapparatus for casting an aluminum alloy engine cylinder block, with acrankcase housing and with seats for cylinder head bolts, in a sand moldaccording to claim 1, further comprising: one of said soluble firstportions being a core forming a mold cavity portion for defining acrankcase housing portion for said block; and others of said solublefirst portions being inserts forming a mold cavity portions at locationsfor said bolt seats.
 10. A method for forming an aluminum alloy castingin a sand mold, comprising: providing a sand mold defining a cavity forforming said casting with at least one first portion of said mold formedwith a binder soluble in a given solvent and at least one second portionformed with binder insoluble in said solvent, said one first portionbeing positioned to define an area of the casting cavity for achievinglocalized rapid cooling of the casting; filling said mold with liquidaluminum alloy; directing at least one cooling stream of solvent onto atleast one first portion to wash out such respective first portion toexpose and directly cool the solidifying casting to give acceleratedcooling of the defined area.
 11. A method according to claim 10, whereineach cooling stream is initiated while the casting is still molten withthe thickness of the respective first portion and the removal thereof bysuch cooling stream being adjusted to have the respective first portionremain intact sufficiently to support the molten casting during initialcooling at the respective defined area as needed, with removal to exposesuch area to direct impingement being accomplished once the casting insuch defined area becomes self-supporting and maintaining saidimpingement on said casting for a time effective for improving thecooling rate and the mechanical properties of said casting at least inthe region proximate to the respective defined area.
 12. A method forforming a large complex aluminum alloy casting according to claim 11,wherein said respective defined area is positioned to achievedirectional cooling remote from and directed towards a source of saidliquid aluminum in a manner to avoid voids and shrinkage porosity.
 13. Amethod according to claim 10 for low pressure casting of an aluminumalloy engine cylinder block in a sand mold having areas defining acrankcase housing and seats for bolts further comprising setting one ofsaid first portions as a sand-core to define the crankcase housing andsetting other first portions as cores to define seats for bolts.